This invention relates to on-machine supercalender apparatus adapted to be located at the output side of a paper machine and which comprise both hard and soft calender rolls which form soft calendering nips with each other. More particularly, this invention relates to on-machine supercalender apparatus wherein the calender rolls are arranged in at least three roll groups each of which consists of three rolls and wherein the roll groups are arranged separate from each other, i.e., the rolls of different roll groups are neither directly nor indirectly in nip contact with each other.
The calendering of a paper web leaving the discharge end of a paper machine is a well known final finishing treatment for determining the smoothness and gloss of the surfaces of the paper as well as its consistency or density. Such calendering is generally accomplished by guiding a continuous paper web successively through a series of nips formed by calender rolls.
Conventionally, a paper web is calendered in a so-called machine calender directly connected to the output side of the paper machine and, when required, the treatment is completed in a separate so-called supercalender.
Calender apparatus comprise calender rolls which define calender nips through which the web is passed. Such calender rolls constitute either "hard" rolls or "soft" rolls. It is understood that as used herein, the term hard rolls designates rolls formed, for example, of chill casting or steel, the hard surfaces of which have been ground smooth. The term soft rolls as used herein designates rolls whose surfaces are made of flexible or resilient material. For example, a resilient material generally used for such soft rolls is paper wrapped in layers around the shaft of the roll and compressed to form a uniform roll coating.
Furthermore, as used herein, the term .cent.soft nip" designates the contact line between a soft roll and a hard roll. The term "hard nip" is used to designate the contact line formed between two hard rolls.
It is possible depending upon the type of paper and the requirements therefor to machine-calender the paper web in a single nip calender, i.e., a calender comprising only one pair of rolls. In most cases, however, a machine-calender will comprise four to eight rolls forming three to seven nips.
It is usually an object in supercalendering operations to provide both sides of the paper web with an equal gloss. Accordingly, at least two soft nips are provided located in a manner such that both surfaces of the paper web are pressed against the surface of a hard roll. Separate supercalender apparatus can comprise up to ten nip pairs.
In connection with improving the efficiency of paper production, it has become important to provide calender apparatus in which both the functions of a machine calender as well as a supercalender are combined. In this connection, Finnish patent application No. 761764 discloses an on-machine supercalender apparatus adapted to be connected to a paper machine. This supercalender comprises a stack of rolls including conventional hard rolls and essentially the same number of soft rolls which are located outside of the roll stack to form soft nips against the hard rolls.
In such calender apparatus which combine the functions of both a machine calender and a supercalender, the paper web can be supercalendered as desired immediately after the same leaves the paper machine without any intermediate phases. However, the results obtained are not entirely satisfactory in that the so-called super gloss obtained by the paper by such calendering treatment is not uniform. In other words, some areas of the surface of the paper are glossier than other areas. Furthermore, it has been found that the paper web subjected to the calendering treatment turns a blackish color resulting at least partially from the fact that the hard nips are in fact too hard and insufficiently flexible or resilient.
As to the state of the art relative to the present invention, reference is made to Finnish Pat. No. 55694 and Finnish patent applications Nos. 793200 and 793201.
Thus, the starting point of the present invention is the observation that soft rolls of a calender, such as the paper rolls described above, are easily damaged during operation. This is disadvantageous in that the ratio of down time to production time becomes quite high.
Conventional calenders are known which are composed of groups of three or more rolls. It has not been possible in such calenders to replace a soft roll when the same is damaged during operation of the machine. In certain conventional four nip calenders, although it is possible to change a soft roll during operation without breaking the web, this results in one surface of the web being totally uncalendered.
As mentioned above, the soft rolls utilized in supercalendering become worn relatively rapidly and frequently become damaged during operation. On the other hand, since the decision to utilize an on-machine supercalender instead of the traditional supercalender depends on the extent to which the output of production of the paper machine is reduced due to the use of the on-machine supercalender located at the output end of the paper machine, it is of primary importance to minimize the duration of interruptions in production incurred during replacement of the calender rolls and especially the soft calender rolls.